Implantable heart valve prostheses have been used to replace various diseased or damaged natural aortic valves, mitral valves, pulmonic valves and tricuspid valves of the heart. The aortic and mitral valves are most frequently replaced due to heart disease, congenital defects or injury. The mitral valve controls the flow of blood between the left atrium and the left ventricle and the aortic valve controls the blood flow from the left ventricle into the aorta. Generally, the known heart valve prostheses are either bioprostheses or mechanical heart valve prostheses.
The bioprostheses or “tissue valves” are generally made of a suitable animal tissue, e.g., harvested swine valve leaflets, mounted onto a stationary metal or plastic frame, referred to as a “stent”. Exemplary tissue valves formed of swine valve leaflets mounted to struts of a stent are those disclosed in U.S. Pat. Nos. 4,680,031, 4,892,541, and 5,032,128 as well as the MEDTRONIC® Hancock II® and Mosaic® stented tissue valves. Some prosthetic tissue valves are formed from treated integral swine valve leaflets and valve annulus structure, e.g. the MEDTRONIC® Freestyle® stentless aortic root bioprostheses
Modern mechanical heart valve prostheses are typically formed of an annular valve seat in a relatively rigid valve body and one or more occluding disk or pair of leaflets that is movable between a closed, seated position in the annular valve seat and an open position in a prescribed range of motion. Such mechanical heart valves are formed of blood compatible, non-thrombogenic materials, typically currently comprising pyrolytic carbon and titanium. Hinge mechanisms and/or pivoting guides entrap and prescribe the range of motion of the disk or leaflets between open and closed positions. Exemplary bi-leaflet mechanical heart valves are disclosed in commonly assigned U.S. Pat. Nos. 4,935,030 and 6,139,575 and in U.S. Pat. Nos. 6,176,877 and 6,217,611.
Mechanical and tissue valves have advantages and disadvantages. By their very nature, mechanical heart valves have metal or plastic surfaces exposed to the blood flow, which remain thrombogenic even long time after their implantation by major surgery. The opening and closing of mechanical heart valve occluders can damage blood elements and trigger a coagulent cascade. Blood flow disturbances in certain mechanical valves are also believed to aggravate blood coagulation. Therefore, patients having such mechanical heart valves can avoid potentially life threatening embolus formation only by taking anti-thrombogenic or anti-coagulent medication on a regular basis. Porcine tissue valves include three cusps or leaflets of a heart valve excised from pigs and preserved by treatment with glutaraldehyde. The preserved porcine tissue is thrombogenic, and therefore, the human patient takes anti-thrombogenic or anti-coagulent medication at least a period of time after the surgical implantation of a tissue valve. Valve leaflet opening and closing characteristics and blood flow past open tissue leaflets of tissue valves can be superior to those afforded by mechanical valves. However, tissue leaflets can become calcified over time distorting the leaflet shape and ultimately leading to failure of the tissue leaflets to fully close or open. Proposals have been advanced to form mechanical heart valve prostheses from flexible, anti-thrombogenic, polymeric sheets or fabrics that are resistant to calcification mounted to stents to function like stented tissue valves also been proposed as exemplified by U.S. Pat. No. 5,562,729. However, calcification and tear issues of polymeric materials remain to be solved before a polymeric valve can be realized.)
Such mechanical and tissue valve prostheses are intended to be sutured to peripheral tissue of a natural heart valve orifice (the “valvar rim”) after surgical removal of damaged or diseased natural valve structure. Modern prosthetic heart valves are typically supplied with a sewing or suturing ring surrounding the valve body or stent that is to be sutured by the surgeon to the valvar rim. Suturing rings typically comprise a fabric strip made of synthetic fiber that is biologically inert and does not deteriorate over time in the body, such as polytetrafluoroethylene (e.g., “Teflon PTFE”) or polyester (e.g., “Dacron”), that is woven having interstices permeable to tissue ingrowth. The valve body or stent is typically circular or ring shaped having an outer surface or sidewall shaped to fit with an inner sidewall of the suturing ring. In some cases, the suturing ring fabric is shaped to extend outward to provide a flattened collar or skirt that can be applied against and sutured to the valvar rim, as shown for example in U.S. Pat. No. 3,997,923.
It is proposed in the prior art to make the valve body or stent rotatable within the annulus of the suturing ring. The surgeon can first suture the suturing ring to the valvar rim and then rotate the valve body or stent within the annulus of the suturing ring in order to adjust the angular orientation of the valve mechanism in the path of blood flow. In this way, the valve mechanism can be rotated to minimize interference with the adjacent heart structure or to divert blood flow past the open valve leaflet(s) in an optimal flow direction. Such rotation of the valve mechanism with respect to and within the annulus of the suturing ring requires a rotational torque sufficiently small as to avoid damage to the sutured valvar rim or loosening of the sutures, and yet sufficiently great so that the valve mechanism, when properly positioned, does not further rotate during chronic implantation. Moreover, the configuration and attachment methods should be such as to provide highly reproducible rotational torques so as to maximize productivity in manufacture and minimize scrap and rework. Configurations and methods of attaching suturing rings to annular valve bodies to satisfy these requirements are disclosed in the prior art, e.g., those described in the above-referenced '240 patent and in U.S. Pat. Nos. 5,071,431, 5,397,346, 5,876,436, 6,113,632 for example. However, none of the current available tissue valves allows a surgeon to rotate the stent within the suturing ring because the suturing ring is an integral part of the stent.
Most suturing rings are formed of a radiopaque stiffening ring or band formed of stainless steel or titanium having an inner annular wall and an outer wall extending between axial ends of the band. The fabric is affixed either to the outer wall of the stiffening band as disclosed, for example, in the above-referenced '632 patent or surrounds the stiffening band as disclosed, for example, in the above-referenced '240 patent. The stiffening band of the '240 is a split band formed with split ends that are drawn against one another during assembly of the suturing ring about the valve stent or body to reduce the inner diameter of the split band in order to overcome difficulties encountered in fabrication employing a continuous ring or band that are described in detail in the '240 patent. The split ends are held together by a cord or heat shrink band or shape memory alloy band that is encased within the fabric when the fabric is sutured together. The interior space within the fabric can be filled with an elastomeric compound.
Separation of the finished suturing ring from the valve body or valve stent to replace the mechanical or tissue valve is not suggested in the '240 patent. Any attempt to do so by cutting the suturing ring assembly apart where the split ends abut one another would effectively destroy the suturing ring. Moreover, it would not be possible to locate the split ends beneath the fabric without disassembling the fabric as well as the cord or band.
Despite improvements in longevity, adverse reactions and complications with implanted mechanical heart valves and tissue valves of the types described above requiring surgical replacement can occur from time-to-time during the lifetime of a patient. It has long been recognized that it is desirable to avoid removing and replacing the suturing ring if it is intact and is not implicated in the adverse reaction or complication. Removal of the existing sutures to remove the suturing ring and restitching of a new suture ring in place can compromise the integrity of the valvar rim and lead to further recovery complications, morbidity and mortality. Therefore, attachment and detachment structures and methods have been proposed to enable the removal of the defective mechanical or tissue valve from the suturing ring and insertion of a replacement mechanical or tissue valve into the annulus of the suturing ring sutured to the valvar rim.
In one approach disclosed in the above-referenced '128 patent, the valve stent is sutured to the suturing ring upon initial implantation. Replacement of the tissue valve involves severing the sutures by a scalpel worked between the suturing ring annulus, withdrawing the released tissue valve, inserting a new tissue valve into the ring annulus and suturing it in place. This approach requires a valve design that can be stitched in place in the suturing ring annulus, does not allow the tissue valve to be easily rotated in the suturing ring in the manner described above, and requires considerable care and time to execute. Pannus overgrowth and calcification of the surfaces at the junction between the valve stent and the suturing ring occur over time that must be cut away or through without damaging the suturing ring. Consequently, other approaches have been devised allowing rotation of the replacement tissue or mechanical valve within the suturing ring annulus.
A detachable suturing ring having a shape-memory member that expands in diameter when cooled below blood temperature to allow release and replacement of a valve body or stent is disclosed by J. Jansen et al. in “Detachable Shape-Memory Sewing Ring for Heart Valves”, (Artificial Organs, vol. 16, No. 3, pp. 294-7, 1992). While this approach would appear to be very beneficial, it has not been established that the temperature induced shape changes are robust enough to assure retention of the tissue or mechanical valve or large enough when cooled by cooling fluid to expand a chronically implanted suturing ring to allow removal of a valve body or stent. Pannus overgrowth and calcification about the suturing ring would likely reduce the amount of expansion or prevent any appreciable expansion of the ring when it is cooled in the attempt to expand it. Moreover, this concept is impractical because of high cost and difficulty in fabrication.
More typically, it has been proposed to use an interlocking mechanism of the suturing ring and the valve body or stent that can be operated by the surgeon to detach or attach valve body or stent from or to the suturing ring. Mating male and female screw threads, snaps, fabric hooks, screws, or other interlocking mechanisms are disclosed in the above-referenced '031 patent, '923 patent, '541 patent, and in U.S. Pat. Nos. 3,997,923, 4,078,268, 4,506,394, 4,705,516, 4,790,843, 6,217,611 and Re.31,040, for example.
These interlocking mechanisms necessarily must be robust enough to ensure that they do not fail during chronic implantation thereby loosening or releasing the tissue or mechanical valve and endangering the life of the patient. The interlocking mechanisms must be large enough to be readily manipulated and to not become frozen due to coagulation or exposure to blood and fluids. On the other hand, the interlocking mechanism must be minute in size in order to avoid taking up space that would reduce the size of the valve annulus or interfere with leaflet function. Consequently, none of these proposed interlocking mechanisms have been adopted.
In the following description and claims, the term “heart valve mechanism” embraces a tissue valve mechanism comprising a stent supporting tissue leaflet(s) and a mechanical heart valve mechanism comprising a heart valve body supporting a pivotal disk or leaflet(s). For convenience, the term “valve frame” means a stent of a tissue valve or a valve body of a mechanical heart valve or equivalents thereof, and the term “occlude” means tissue leaflets of a tissue valve or pivotal disk or leaflets of a mechanical heart valve or equivalents thereof. The assembly of a tissue or a mechanical heart valve mechanism with a suturing ring can be characterized as a heart valve prosthesis.
Thus, there remains a need for improvements in suturing rings that facilitate the initial implantation and replacement of chronically implanted heart valve mechanisms supported by the suturing rings. Among other things, there remains a need for suturing rings that are separable from the heart valve mechanisms that can accommodate a variety of heart valve mechanisms including either tissue valves or mechanical heart valves of differing designs and that are highly versatile in use including potential use in minimal invasive surgery techniques. There remains a need for such suturing rings that have a locking mechanism that does not add bulk to the suturing ring or diminish the suturing ring and valve annulus, that are simple to manufacture and use, and that are robust, durable and reliable. There remains a need for a separate suturing ring that facilitate surface modification of the suturing ring fabric without affecting tissue valve leaflets and that is free of glutaraldehyde employed to preserve the tissue leaflets prior to implantation.